In typical cellular radio telephone systems, the area served is divided into hexagonal cells wherein each cell is defined by a radio base station (RBS) having a plurality of transceivers for communicating both control and voice information with mobile units, i.e., mobile telephones. As long as a mobile unit is powered-up within the broadcast range of any RBS defining a cell of the system, the mobile unit is capable of initiating and receiving calls. A centralized mobile switching center (MSC) connected to all of the RBSs connects the mobile unit's outgoing and incoming calls to and from the public telephone network and to other mobile units.
When a mobile unit initiates a call in a system, it uses a control channel (i.e., a reserved carrier frequency) assigned to the RBS that defines the local cell in which it is currently located. When any two-way communication between the mobile unit and the RBS takes place on that particular control channel, the identity of the cell in which the mobile unit is currently located is known to the system. To handle voice communication, the system generally assigns a voice channel from a plurality of RBS transceivers available in the same cell that corresponds to the control channel.
While engaged in a call, the mobile unit often moves beyond the broadcast range of the current cell into the broadcast range of another cell. In this case, the system ordinarily transfers the mobile unit to a voice channel available in the new cell. Accordingly, while engaged in a call the system knows the current cell location of a mobile unit.
Even while not engaged in a call, an activated mobile unit receives information sent by the system on the control channel of the cell in which the mobile unit is currently located. When the mobile unit moves beyond the broadcast range of the current control channel, it is designed to search for and re-tune its receiver to the control channel of the cell serving the mobile unit's new location. However, as the mobile unit moves from one cell to another it typically does not transmit information to the new cell. The system therefore does not know the current cell location of a mobile unit that is neither engaged in a call nor in the process of initiating a call.
Accordingly, when an incoming call arrives at the MSC for a mobile unit, the MSC ordinarily does not know the current cell location of the mobile unit nor does it know the cell control channel to which the mobile unit is currently tuned. In order to determine the cell in which the mobile unit is currently located, the MSC sends control information addressed to that mobile unit on the control channel of every cell in which the mobile unit could possibly be located. When the mobile unit recognizes its address on the control channel to which it is currently tuned, it responds to the system on the control channel of that cell and thereby notifies the system of the cell that is currently serving the mobile unit. In other words, the MSC pages the cells of the system until the mobile unit is located.
Although locating a mobile unit by sending information to the mobile unit on the control channel of every cell is an acceptable procedure in systems serving a small number of mobile units, this procedure becomes very inefficient as the number of mobile units increases and the number of cells increases to serve larger geographic areas or to provide more call capacity. At some point, the capacity of the control channels becomes exhausted due to a large portion of that capacity being used in an attempt to locate mobile units, even though each mobile unit is only present within one of the many cells of the system at any given time.
To overcome this capacity problem, current systems employ several techniques to attempt to reduce the number of cells into which this redundant information must be sent. These methods are all directed to methods of determining the probable location of the mobile unit within a subset of all possible cells by having the mobile unit report its location even if it does not need to communicate with the system for other purposes such as originating a call. By way of example, two such methods include 1) periodic or time-based notification, and 2) location-based notification. In addition, both of these methods may be combined with notification (of the mobile unit's presence in the system) on power-up and power-down of the mobile unit to reduce attempts to locate mobile units that are not currently powered-up in any cell of the system.
In the time-based notification method, the system sends information on the control channel of each cell indicating when one or more of the mobile units should notify the system of their location. The system can then determine the probable location of a mobile unit based on the time and location of the last report and the anticipated rate of movement. This method is particularly effective in systems covering very large areas with large cells. However, when the cells are relatively small or the rate of movement is relatively fast, the time interval between reports must be shortened to accommodate the frequent cell location changes. As a result, the proportion of location reports to incoming calls increases, thereby burdening the system with frequent, unnecessary reports which tend to overload the available control channels. This method also burdens the system with periodic reports from mobile units that have not changed their location.
In the location-based reporting method, cell identification information is sent by each cell on its control channel. The mobile unit reports its location whenever it moves to a cell control channel that is sending different cell location identification information. One well-known type of cell identification occurs when a mobile unit moves from one cellular system to a different system, and the mobile unit receives the system change information and reports its location to the new system.
One technique for location-based reporting within the same system includes transmitting location information common to a fixed group of cells, i.e., to a subset of the total number of cells grouped into fixed location areas. Typically, the cells are geographically organized into groups of cells having fixed grouping boundaries. In this case, each fixed group of cells typically transmits the same group location identification to mobile units therein. When a mobile unit moves from one group location to another, it reports its new group location to the system. With this method mobile units report their location only when and if they move from the previously reported group location, thereby eliminating unnecessary location reports and allowing the system to know the mobile unit's location to within a fixed group of cells.
The number of fixed location areas, and the number of cells in each location area can be adjusted to balance the overhead of registrations (caused by the mobile unit's movement and resultant reporting) against the overhead of paging multiple cells of the group. In other words, as a fixed group increases in the number of cells that comprise it, the greater the amount of paging required to find the mobile unit within its current group, while as a fixed group decreases in the number of cells that comprise it, the greater the amount of location reporting resulting from more numerous and frequent crossings of the fixed boundaries.
While this system is often able to reduce the system overhead by reducing the total amount of paging in exchange for location reporting, problems arise since the location areas are defined by a set of cells sending common location identification to all mobile units receiving the control channels of these cells.
By way of example, when two adjacent cells in different fixed location areas are sending different location identification information across a fixed boundary, the mobile unit will report its location each time it moves from one cell to the other. Since a mobile unit may repeatedly move between cells in a short period of time, the number of location reports will be excessive. In addition, such numerous registrations tend to quickly drain the batteries on certain types of mobile units (for example, hand-held mobile units with a self-contained power source as opposed to mobile units powered by car batteries) since signal transmission ordinarily consumes more power than does signal reception.
Moreover, substantial problems can arise when the mobile unit travels across boundaries of different systems. Essentially, different adjacent systems are somewhat analogous to fixed location areas in that the mobile unit must report its location to the new system upon entry in order to be found by the MSC that receives its incoming calls.
For example, when a mobile unit changes systems, such as when moving from a "home" system to a "foreign" system, the foreign system may be arranged to report the presence of the mobile unit therein to the home system. In this manner, the home system is able to forward incoming calls to the foreign system. However, there is often a time delay between the time that the mobile unit actually enters the foreign system and the time that the home system receives notification of the mobile unit's presence therein. If the mobile subsequently reenters a cell in the home system during this time delay, i.e., prior to the home system receiving the notification, it re-registers its presence with the home system. When the home system later receives the notification from the foreign system, the home system concludes that the mobile unit has moved into a cell of the foreign system when in actuality it has just returned. In essence, under these circumstances the location reports are received out of sequence, thereby resulting in the recording of incorrect location information. This results in the mobile unit being "lost" to the home system until a subsequent location registration takes place.